1. Field of the Invention
The present invention relates to a golf club head and, more particularly, to scorelines in the face.
2. Description of the Related Art
In general, a plurality of linear grooves are formed in the face of a golf club head so as to run parallel to each other in the toe-to-heel direction (see, for example, Japanese Patent Laid-Open Nos. 10-248974 and 2005-169129). These grooves are called, for example, scorelines, marking lines, or face lines (these grooves will be referred to as scorelines in this specification). These scorelines have the effect of increasing the amount of backspin on a struck golf ball, or suppressing a significant decrease in the amount of backspin on a struck golf ball upon a shot in rainy weather or that from the rough.
A rule concerning the scorelines of an athletic golf club head stipulates that the edge of a scoreline must fall within a virtual circle which has a radius of 0.011 inches and is concentric with a virtual circle which has a radius of 0.010 inches and is inscribed in both the side wall of the scoreline and the face (to be referred to as the “two-circle rule” hereinafter). This two-circle rule admits, as an exception, a scoreline which has its edge falling outside a virtual circle with a radius of 0.011 inches but which satisfies the condition in which the included angle between two segments which connect the center of the virtual circle and two intersections between the virtual circle and the contour of the edge of the scoreline falling outside the virtual circle is 10° or less (to be referred to as the “maximum angle rule” hereinafter). However, the edge of the scoreline is prohibited from projecting in excess of 0.0113 inches (0.287 mm) from the center of the virtual circle (to be referred to as the “maximum projection rule” hereinafter).
To satisfy the two-circle rule, it is basically inevitable to set a relatively narrow angle of the side wall of a scoreline with respect to the face. This decreases the amount of spin on a struck golf ball. Conversely, when the angle of the side wall of a scoreline with respect to the face is set relatively wide, this leads to violation of the two-circle rule unless a measure involved is taken.